This invention relates to tin-free steel strips useful in the manufacture of welded cans finding applications as food cans, beverage cans, 18-liter cans, pail cans, and other commercial cans.
The most widely used can-forming materials are generally tinplate and tin-free steel. For the reasons of resource saving, cost saving, and appearance, tinplate cans have been progressively converted from soldered cans to welded cans. At the same time, the weight of tin coating has been reduced, that is, tin plates having as thin as 1.0 g/m.sup.2 or less of tin coating have been developed instead of conventional heavily plated ones having 2.8 g/m.sup.2 or more of tin. From the standpoint of economy, however, it does not appear that lightly coating tinplate is superior to tin-free steel. This is one of the reasons of the increasing use of tin-free steel.
Despite the economic advantage, the tin-free steel suffers from a severe problem. More particularly, tin-free steel strips are steel strips having thin coatings of metallic chromium and chromium compounds (usually hydrated chromium oxides) on the surface. In order to avert its own drawbacks that it can be neither soldered nor welded because of the high electric resistance and high melting point of the surface coatings, tin-free steel is, for the most part, used as cemented cans.
Such cemented cans, however, encounter the trouble of can barrel rupture. That is, cemented seals can be broken during high temperature sterilization of can contents. Irrespective of some recent improvements accomplished by the modification of the hydrated chromium oxide coating of tin-free steel, cemented cans are always liable to such a danger. If a weldable tin-free steel strip is developed, not only the rupture trouble could be avoided, but the overlapping distance at a bond could be reduced from about 5 mm required for cementing to about 0.2 to 0.4 mm for welding, leading to a material saving. Also, the risk of vacuum leakage from crimped portions can be prevented. Thus there is a great need for the development of a weldable tin-free steel strips.
Weldable tin-free steel strips and processes for their preparation are known in the art as disclosed in Japanese Patent Publication Nos. 57-19752 and 57-36986. These prior art techniques improve weldability by reducing the amount of metallic chromium or chromium compounds at the sacrifice of corrosion resistance because the resultant metallic chromium layer on such tin-free steel inevitably becomes porous in structure.